The invention relates to articles based on polyolefins which are printed by electrophotography by means of toners, and to a process for the manufacture thereof.
In numerous applications, articles consisting of plastic, and in particular films, sheets and hollow articles, are printed by means of conventional contact printing techniques such as screen, gravure or flexographic printing, in which a plate carrying in relief the design to be reproduced is inked and then pressed against the substrate to be printed. The inks used in these processes are inks based on solvents in which dyes are dissolved. Techniques for treating the surface of the substrates have also been developed for the purpose of improving the adhesion of these conventional inks to various substrates. For instance, in European Patent Application EP 678398 (Solvay), it is indicated that the oxyfluorination of polyolefinbased articles under specific conditions allows them to be printed using inks for PVC, which is not possible in the absence of this surface treatment.
Recent years have, however, seen the development of contactless printing techniques, an example being the technique of electrophotography. This technique, which is widely used in computer-controlled digital printers, uses inks which are completely different from those used in the contact printing techniques, and which are generally referred to as xe2x80x9ctonersxe2x80x9d. These toners are fine powders which in a first stage are deposited on the substrate to be printed and in a second stage are melted, so causing them to adhere to the substrate (an example being the XEIKON(copyright) process). These toners, which are generally polymer-based, have characteristics which are entirely different from those of conventional inks, and are not suitable for the same substrates. For instance, the results of attempting to print a polyolefin film by electrophotography, even after corona treatment, are very poor, in the sense that the adhesion of the toner to the film is virtually zero. As a result, the manufacturers of polymer substrates intended for this technique have, in general, to coat their substrates with a thin coating layer that is suitable for printing, based for example on polyethylene terephthalate (PET) or on polyamide. The production of such a coating naturally results in a considerable increase in the cost of the substrate, and may in certain cases impair some of its mechanical, visual or other properties.
The present invention aims to overcome these drawbacks by providing a polyolefin-based article which can be used in processes for printing by electrophotography by means of toners, and which is easy to manufacture and inexpensive.
A first subject of the invention is therefore a polyolefin-based article which is printed by means of a toner by an electrophotographic technique and makes it possible to obtain good adhesion in the absence of a specific printable surface coating.
More specifically, the invention provides an article of which at least one surface zone comprises at least one polyolefin and has been printed by means of a toner by an electrophotographic technique, characterized in that the printed zone contains no toner printing primer and in that the adhesion of the toner to the article is such that it withstands a stripping force of at least 0.4 N/mm exerted by means of an adhesive tape.
The articles referred to within the scope of the present invention can be of any type, in particular films, sheets or plates, or else hollow articles such as bottles, drums, tanks, flasks, pipes, etc. The invention is particularly advantageous in the case of flat articles, especially in the case of films. These flat articles can be produced by any means, in particular by calendering, by extrusion or by coextrusion, for example by extrusion blow-moulding, extrusion lamination, flat-die extrusion, and by similar coextrusion techniques. In accordance with the invention, at least one surface zone of the article must comprise at least one polyolefin. Preferably, this zone essentially consists of at least one polyolefin. One or more other parts of the article may consist essentially of one or more other materials, such as a metal or a cellulosic material. The invention applies to monolayer and multilayer articles. Thus, for example, the invention applies inter alia to multilayer articles of which at least the printed surface layer is based on polyolefin, it being possible for one or more other layers to consist essentially of one or more other materials.
The term polyolefins is intended to denote not only olefin homopolymers but also copolymers containing at least 70% of olefin-derived units, and also any mixture of such homopolymers and/or copolymers. The term olefin is also intended to denote monoolefins such as ethylene, propylene or butene and olefins containing more than one double bond, for example diolefins such as butadiene. Non-limiting examples of polyolefins which may be mentioned are the polymers of propylene and of ethylene. Interesting results have been obtained with articles whose treated and printed surface zone consists essentially of a propylene polymer or of a mixture of from 50 to 99% (relative to the total weight of the polymers) of at least one propylene polymer and from 50 to 1% of at least one ethylene polymer. The term propylene polymer is intended here to denote a homopolymer or a copolymer containing at least 70% by weight of propylene.
This polyolefin or these polyolefins may further be admixed optionally with one or more conventional mineral fillers, such as calcium carbonate, titanium dioxide, mica; reinforcing fibres such as, for example, glass fibres or carbon fibres, and one or more conventional additives, such as stabilizers, lubricants, antioxidants, etc.
In addition to one or more polyolefins, fillers and additives as set out above, the articles in question may optionally include one or more other polymers intended to give them specific propertiesxe2x80x94for example, for the purpose of improving their impact strength.
In accordance with the invention, the articles are printed by means of a toner. The term xe2x80x9ctonerxe2x80x9d is intended to denote any solid (pulverulent) ink suitable specifically for printing by electrophotography. In general, these toners, which are well known to the person skilled in the art, contain primarily a thermoplastic resin, a colorant, a charge control agent and, optionally, a magnetic powder (as well as, optionally, certain other additional additives).
The majority constituent of the toner is a thermoplastic resin capable of acting as a binder for the other constituents, such as, in particular, an acrylic polymer [e.g. a copolymer of alphamethylstyrene with alkyl (meth)acrylates], polyesters, or epoxy resins.
As the charge control agent use is made, for example, of nigrosines (or nigrosine derivatives), metal salts of higher fatty acids, alkoxylated amines, certain quaternary ammonium salts, or alkylamides. The amount thereof is generally from 1 to 10 parts relative to the weight of the thermoplastic resin.
Any appropriate colorant can be used, especially carbon black, a dye of the nigrosine type, ultramarine blue, etc. The amount thereof is generally from 1 to 20 parts relative to the weight of the thermoplastic resin.
As other optional additives, mention may be made, for example, of natural or synthetic waxes, fluorinated resins or silicone resins, silica particles, etc.
The optional particles of magnetic powder consist of a ferromagnetic alloy or metal.
A number of ingredients from these various categories may optionally be used in combination.
These various ingredients are generally mixed and ground before being pulverized to give a powder of appropriate particle size (generally of the order of from 0.1 to 5 xcexcm).
Examples of such toners, of their constituents and of the process, for their preparation are provided in particular in the U.S. Pat. Nos. 4,840,863 and 4,299,898.
Processes for printing by electrophotography (xerography) are well known per se, as described for example in the U.S. Pat. Nos. 3,618,552, 2,874,063, 3,251,706, 2,221,776, 3,166,432, 2,986,521. They generally comprise electrically charging certain zones of a cylindrical drum (formation of a latent image), depositing thereon the toner, which has been charged beforehand with an inverse polarity and which will attach itself only to the charged points of the drum, subsequently transferring the toner from the drum to the substrate to be printed, and finally causing it to adhere to the substrate by thermal initiation of the melting of the thermoplastic binder. In one variant, the electrostatic attraction can be replaced by magnetic attraction.
The term toner printing primer is intended here to denote a surface coating which is applied by coating to the surface of the article, whose thickness is of the order of from 0.1 to 10 xcexcm, and which is suitable for printing by a toner. A coating of this kind is considered to be suitable if a substrate thus coated, printed by electrophotography by means of a toner, withstands an adhesive tape stripping test with a force of at least 0.4 N/mm, i.e., if the toner continues to adhere to the said substrate without being transferred to the tape despite the said force being reached. In this test, which is carried out at a temperature of approximately 23xc2x0 C., the tape is pulled off horizontally, i.e. forming an angle of 360xc2x0, at a rate of approximately 100 mm/min. This same test is used to characterize the articles in accordance with the invention.
These primers are generally based on polar polymers (although they may optionally comprise a small amount of polyolefin(s)), for example polyamides, PET or acrylic polymers, and are optionally filled (for example with silica).
The polyolefin-based printed articles thus defined differ from those known to date in the excellent adhesion of the toners, this adhesion being obtained despite the absence of a printing primer coat.
Preferably, the printed surface zone has been surface-treated by means of oxygen and fluorine before its printing.
Advantageously, the said surface zone contains fluorine and oxygen at the surface in concentrations such that the atomic ratio of oxygen to carbon (O/C), measured by ESCA spectroscopy at a depth of 1.5 nm, is at least 0.06 and that the atomic ratio of fluorine to carbon (F/C) has a value of at least 30% of that of the ratio O/C and not more than 350% of this ratio.
In the treated surface zone, the atomic ratio O/C is generally less than 0.40 and preferably less than 0.30. Advantageously, the atomic ratio F/C is greater than 50% of the ratio O/C.